Lamont was born in Claverack, New York. His parents were Thomas Lamont, a Methodist minister, and Caroline Deuel Jayne. Since his father was a minister, they moved around Upstate New York a lot and they were not very wealthy.[1] He graduated from Phillips Exeter Academy in 1888, where he was editor of the school newspaper, The Exonian, as well as the school yearbook and literary magazine. He then attended Harvard.[1]

At Harvard College, he became first freshman editor of The Harvard Crimson, which helped him pay off some of his tuition. He graduated cum laude with a Bachelor of Arts degree in 1892. He met his wife, Florence Haskell Corliss, at the 1890 Harvard commencement. He started working under the city editor for the New York Tribune two days after he graduated from Harvard in 1892.[2] He married Florence on October 31, 1895 in Englewood, New Jersey.

At the Tribune, he received many promotions, including night editor and helping the financial editor, which gave him his first taste of the financial world. He left journalism because of the low pay and went into business[1]

He began working in business for Cushman Bros., which later became Lamont, Corliss, and Company, and turned it into a successful importing and marketing firm. It was an advertising agency that worked for food corporations. The company was in a bad financial status, but Lamont fixed it, and the company changed its name to Lamont, Corliss, and Company. He was partners with his brother-in-law, Corliss. His banking caught the attention of banker Henry P. Davison, who asked Thomas to join the new Bankers' Trust. He started as secretary and treasurer and then moved up to being Vice President and then was promoted to director. He rose to the vice presidency of the First National Bank.[1]

In 1918, he purchased the New York Evening Post, of which his brother, Hammond, had been managing editor a decade earlier, from Oswald Garrison Villard. After failing to make a profit, he sold the paper in January, 1922 to a syndicate that was headed by the paper's editor, Edwin F. Gay.

The company had also started an improvised system so that the Allies could buy supplies from them. In 1917, he joined the Liberty Loan Committee, which helped the treasury sell war bonds to Americans. He also served unofficially as an advisor to a mission to the Allies, led by Edward M. House, as requested by President Woodrow Wilson.[3]

Lamont not only advised the other countries but also went to them. Right before he was going to go to Europe, the Bolsheviks took power in Russia. He and the head of the American Red Cross, William B. Thompson, along with the approval of the British prime minister, Lloyd George, tried to convince America to aid the Bolsheviks so that Russia would stay in the war. However, they were unsuccessful.

In the interwar period, he was a spokesman for J.P. Morgan because J.P. Morgan Jr. was retiring. He handled the press and defended the firm during hearings like the those of Arsene Pujo that investigated powerful Wall Street bankers.[1]

Lamont later undertook a semiofficial mission to Japan in 1920 to protect American financial issues in Asia. However, he did not aggressively challenge Japanese efforts to build a sphere of influence in Manchuria;[4] indeed, he supported Japan's non-militaristic politics until late into the 1930s.

Ron Chernow won the National Book Award for his book The House of Morgan in which he claimed that Lamont had authored the infamous Japanese response to deceive the world about the Mukden incident, which was used as a pretext for Japan's invasion of Manchuria. That defied the expressed position of US government and the League of Nations of Japan, not China, being the aggressor.

Lamont was the chairman of the International Committee of Bankers on Mexico for which he successfully negotiated the De la Huerta-Lamont Treaty. He continued to chair the committee into the 1940s by a series of renegotiations of Mexico's foreign debt.

On September 20, 1940, the fascist police shocked Lamont by arresting Giovanni Fummi, J.P. Morgan & Co.'s leading representative in Italy.[2] Lamont worked to secure Fummi's release. Fummi was released on October 1 and went to Switzerland.

On Black Thursday in 1929, Lamont was acting head of J.P. Morgan & Co. In an attempt to stop the panic, he organized Wall Street firms to inject confidence back into the stock market through massive purchases of blue chip stocks.

A grandson, Lansing Lamont, was a reporter with Time Magazine from 1961 to 1974. He published several books,[7] including You Must Remember This: A Reporter’s Odyssey from Camelot to Glasnost about his experiences covering the important events of the time, including the assassination of Robert Kennedy. Another grandson, Thomas William Lamont II, served in the submarine service on the USS Snook (SS-279) and died when the submarine sank in April 1945.